In the area of motor vehicle control, operating variables are adjusted via actuators subjected to friction. An example is the adjustment of a friction-burdened actuator, such as a throttle flap, in the context of the following: power control, rpm control or a drive-slip control. A corresponding control system is disclosed in U.S. Pat. No. 5,144,915. Difficulties in carrying out the control result from the friction which burdens actuators such as static friction. These difficulties are overcome by limiting the change of the speed of the control variable by intervening in at least one constant of the controller. Such a controller is, however, not suitable for all applications.
In general, the following problems occur in the control of an actuator subjected to friction.
If the system is in a steady state (actuator at standstill), then the drive of the actuator must develop a torque change when there is a change of state. This is necessary to overcome the static friction of the actuator. The torque generated by the drive must therefore be relatively large because, in the opposite case, the actuator does not immediately track the change because of the static friction. Especially for slight changes, the necessary torque change, which overcomes the static friction, takes place only after the elapse of a specific time when the time-dependent components of the controller (especially its I-component) have formed a corresponding control signal. This can lead to disadvantages in the control performance.
A further problem which occurs in addition to the first-mentioned problem or occurs independently therefrom, relates to the transient effect of the operating variable to the desired value. A high torque on the drive of the actuator imparts movement thereto. The static friction becomes sliding friction so that less torque is needed. The torque, which is built up to break loose the actuator from the static friction, can be so high that the actuator shoots beyond the desired position. An effect of this kind is unwanted with respect to the control performance and the driving comfort of the motor vehicle.